The present invention relates to an electric vehicle controller and, more particularly, the invention relates to an electric vehicle controller which is suitable for use in an automobile which is required to frequently perform regeneration of the electrical power source and to have an energy saving driving capability.
As for power controllers for controlling three-phase alternating current, there are a control method in which current is fed back using a stationary coordinate system and a control method in which current is fed back using a rotating coordinate system agreeing with a magnetic flux in an alternating current motor. As for these current control systems, the former method has been widely used because it does not require complex calculation, such as a transformation of the coordinate system. An example of such a current control method for an electric vehicle is disclosed in Japanese Patent Application Laid-Open No. 5-153705. On the other hand, since the latter method can deal with current as a direct current value, the latter method is suitable for a digital controller which has a limitation in response due to the sampling time. Especially, the latter method has an advantage when the primary frequency of the alternating current motor is several hundreds Hz or higher and a digital control method is employed.
In regard to electric vehicles, since one of the most important problems is to attain small size and light weight, a most advantageous controller employs a totally automated digital method using a single chip micro-computer. As for methods of attaining a small sized motor, a high speed motor is advantageous, and accordingly a high primary frequency is required. Therefore, it is expected that the latter method of current control will be widely used in the future.
A second control method known in the art is proposed in Japanese Patent Application Laid-Open No. 59-169369. According to this method, current control can be performed more stably than the previous technology known in the art by integrating a q-axis voltage with a d-axis current difference and integrating a d-axis voltage with a q-axis current difference corresponding to an angular frequency used in the transformation of the coordinate system, respectively. However, there is a problem with this in that the control response largely changes due to a large change in the width of the integral gain, since the angular frequency .omega. largely varies. For example, in a case where the rotation of a motor rapidly changes from a high speed state of 10000 rpm to a low speed state of 100 rpm, correction of a voltage integration value calculated during the high speed-state requires a correction time which is 100 times as long as the correction time required in a normal state.
For this reason, most products generally employ the second conventional control method in which a motor can be driven in the ordinary operating condition without a problem.
However, a motor controller for an electric vehicle is subject to the following conditions compared to a common motor controller. (1) Regenerative operation is sometime performed for a long time period. (2) Comparatively weak magnetic field control is often performed during low driving torque because improvement of the efficiency is most important in an electric vehicle. For example, the motor is driven under the above condition when the vehicle runs on a very long descending road. In the current control method described in the above-mentioned second conventional technology, the stability of the control system is reduced when the vehicle is running on a descending road at a high speed.
This phenomenon can be explained as follows. FIG. 15 is a vector diagram showing a voltage and a current of an induction motor when an electric vehicle runs in a regenerative driving state, under a weak magnetic field control and at a high speed. Considering a vector diagram in connection with integral operation, an integral correction vector V.sub.2* is additionally applied to the difference between a current command vector i.sub.1* and a current vector i.sub.1*, that is, a current difference vector .DELTA.i.sub.1* in the same direction. In this case, since an applied voltage vector V* is smaller than an initial voltage command vector V.sub.1*, the current vector i.sub.1 does not approach the current command vector i.sub.1*, but often moves far from the current command vector i.sub.1*. It has been determined that there are some cases where the stability of a controller for an electric vehicle is extremely degraded particularly when the running condition of the vehicle satisfies the three conditions of high speed, regeneration and weak magnetic field control at the same time.